There are many applications where it is desirable to have a fixed speed motor provide a variable speed output. For example, in ventilation systems, an ac synchronous motor is used to rotate an air mover, such as a fan. The energy efficiency of this system increases if the speed of the motor remains fixed while the speed delivered to the fan can be varied.
In the prior art, there are variable speed drives that mount onto the output shaft of the motor. Around the outer circumference of the drive are one or more sheaves. The sheaves receive belts that are coupled to a load. The drive permits a controlled amount of slip. At zero slip, the full rotary power of the motor output shaft is applied to rotate the sheaves. At full slip, the output shaft continues to rotate, but the sheaves remain stationary under a load. Thus, at zero slip, the fully rotary power of the motor is applied to the load, while at full slip, no rotary power is applied to the load.
In Albrecht et al., U.S. Pat. No. 4,400,638, there is disclosed a shaft mounted eddy current drive. The amount of slip is controlled electrically using eddy currents. The output sheaves are mechanically coupled to poles of an electric coil. There are provided opposite, interdigitated poles. An armature provides a magnetic path between the opposite poles. The armature is mechanically coupled to the output shaft of the electric motor. As the motor shaft rotates, the armature also rotates at the same speed as the shaft. In order to rotate the sheaves, current is applied to the coil. This creates an electromagnetic coupling between the poles and the armature, wherein the armature causes the poles and the associated sheaves to rotate.
It is desired to improve upon the prior art drives. The slip rings of the Albrecht et al. drive have the same diameter as, and are located adjacent to, the sheaves. One disadvantage of the slip ring arrangement is the wear on brushes. The chief complaint among customers who buy the prior art drives is brush wear. The brushes must be frequently replaced, adding to the maintenance cost of the drives. The larger the circumference of the slip rings, the shorter the life of the brushes becomes because for each revolution of the motor, the brushes are in frictional contact with a long length of the slip rings.
Another disadvantage of the slip ring arrangement of Albrecht et al. is that as the drive is sized larger or smaller for respective larger or smaller load applications, the circumference of the slip rings change. Thus, the slip rings must be custom made for each size drive. It is desirable to make the slip rings a more uniform size, regardless of the size of the drive, in order to manufacture and repair the drives more efficiently.
Still another disadvantage of the slip ring arrangement of Albrecht et al. is the difficulty in protecting the slip rings and brushes from the environment. If the drive is used outside, it is subjected to moisture, which can reduce the life of the brushes.